Japanese Patent Application Laid-Open Publication No. 2011-250185 (Patent Document 1) discloses a network system in which an inter-device link aggregation is set on each link between one network device and two network devices. When a fault occurs on a dedicated line connecting two network devices, an alternative path is established by using the link aggregation path. Also, two network devices are operated as an active system and a standby system for a control plane such as synchronization of path information between the devices, and are both used in an active state for a data plane.
Japanese Patent Application Laid-Open Publication No. 2012-209984 (Patent Document 2) discloses a configuration in which an inter-device link aggregation is set on each link between a customer edge in a user network and two provider edges in a MPLS network. When both of the two provider edges receive a packet from a different provider edge, only one of the two provider edges relays the packet to the customer edge based on a rule made in advance between the two provider edges.
Japanese Patent Application Laid-Open Publication No. 2012-231223 (Patent Document 3) discloses an access system in which a link is provided each between a user L2 switch and an active L2 switch and between the user L2 switch and a reserve L2 switch. In a normal situation, the reserve L2 switch controls a port serving as a connection source of the link to the user L2 switch to a link-down state. The user L2 switch transmits a broadcast frame such as an ARP to the active L2 switch and the reserve L2 switch, thereby automatically establishing a path bypassing the port controlled to the link-down state in the reserve L2 switch.